Behind closed doors, some couples experience other changes. Changes they don't discuss with the in-laws while debating entrée options. Often, their sex lives become surprisingly hot—and not just for a few short days after the proposal. There's the cliché that a guy forking over the ring starts the slow slide to sexual doldrums; it does happen for some. But many men find that getting engaged actually adds sizzle to their bedrooms. Here's why.

He's On A High.

When we first fall in love, thinking of our partner leads us to produce dopamine, says Sallie Foley, director of the Center for Sexual Health at the University of Michigan. Our brain releases the hormone when we do something novel and fun, like taking a trip. In turn, that dopamine shot brings pleasure and raises our testosterone levels—one reason new couples can't keep their hands off each other.

This mechanism of brain chemistry helps fuel the excitement in engagement sex—just like it does in hotel sex, where the novelty of new surroundings triggers our dopamine jets. It's also a reason your newly engaged guy may suddenly be getting more female attention. Not only is he officially off the market, and thus all the more desirable, but his elevated testosterone levels also may give him a more masculine aura and greater confidence, which women tend to pick up on. Sensing this can make him feel studlier and more self-assured, translating to a bolder approach in the bedroom.

He Feels Like An Adult.

For some men, settling down and no longer "playing the field" can mark the end of a protracted adolescence; he now sees himself as mature and ready for the responsibilities of marriage. This can instill a sense of satisfaction and stability that can be sexy—and make him more relaxed in bed.

"When you know that this is it for you, you put all your energy into a relationship, and you're focused on that," says Lorenzo Joyce, of Lindenwold, NJ, who popped the question in August 2007. Sex has a deeper mental connection for him now, Lorenzo says, even though he and his fiancée were living together before he proposed.

"Getting engaged is a developmental milestone for a man. It marks the end of the swordsman' (he tries to bed all the women he can) phase and the beginning of the next, as a grown man who can commit," explains Boston-based sex therapist Aline Zoldbrod, Ph.D. Men react differently. Some, like Lorenzo, feel a sense of validation and lust-generating machismo, embracing the right to enjoy sex fully, without hang-ups and inhibitions carried for years. For other men, the realization that bachelorhood is over weakens their sexual desire, notes Dr. Zoldbrod.

Sure, this can be simple commitment fear. Or he may feel like he's lost the thrill of the chase. "I've seen men who've had their desire go down after getting engaged, and it's because they're conflicted about what they might have to give up," says Dr. Zoldbrod. It's not just the prospect of not sleeping with other women. "Inwardly, they're afraid they may be forced to relinquish a dream, like having to be a financial planner when they wanted to be an artist." Also, several men (and women) Dr. Zoldbrod has counseled with a post-engagement drop in libido have divorced parents, she notes; there may be some underlying fear they'll face the same fate. Discussing these issues compassionately can help diffuse them, and they may also fade once he enjoys the undeniable perks of committed coupledom.

Sex Can Be Spontaneous.

For couples who move in together once engaged, the newfound convenience of hanging out without having to plan dates can give their sex life a boost. Sex now can happen at any moment.

"Instead of seeing each other twice a week, Charlotte and I were now seeing each other every day," says Joe Singer, of York, PA. "Sex got better because we didn't have to try to fit as much stuff in during a date." There was more freedom, too, without Joe's roommate around. "On moving day," he says, "when there were nothing but boxes in the place, we made love on the bedroom floor."

He's Got Your Enthusiasm.

Your post-engagement high may be contagious. "Monica is very excited about getting married, and it's made her less inhibited," says William Vanyo, of Las Vegas, who proposed in February 2007. "She's much more descriptive about what she wants, and our sex life now involves several purple sex toys," he reveals.

Also, William notes, she's started teasing him in public. "She'll brush her breasts against my arm when we're in a bookshop," he says. "It's very erotic."

You Both Feel More Open About Sharing Fantasies.

Popping the question last Christmas day made Scott Packard, of Altoona, PA, comfortable enough to take the lead romantically. Alison loved the change. When Scott suggested that they hop in the shower together one evening, she was thrilled. Soon she proposed that they watch a risqué DVD. "I was like, Yeah!'" he says. Trekking to the adult store together to choose some toys, however, beat all. "It made me feel like we had a close connection," Scott says. "Maybe we became even more comfortable about our sexuality, knowing we're going to spend our lives together."

His Tension Has Dissipated.

The period just before the engagement takes place can be difficult for a couple, even if they've picked out the ring together. As time passes, questions can multiply in a woman's mind. "Why hasn't it happened? Is it ever going to?" If he senses this impatience, "He may feel pressured and think, She's not going to rush me on this,'" says Foley. This tension can put a damper on sexual energy. Once the proposal occurs, breaking out of this conflict and ambiguity can be freeing. "You've picked your path and made a decision," she says—and the clarity and focus that comes with this can make you both feel energized and elated.

Stacey Pierce-Scott, of Towson, MD, experienced this with her fiancé, Randolph. "Whenever we would talk about marriage, he would tap his leg constantly and get stressed," she recalls. "It would often turn into a heated discussion." The conflict took a toll on their intimacy. "Our sex life was suddenly half as exciting," she says.

After Randolph proposed, the pressures were finally off his mind—and it brought instant relief. "The moment we got engaged, he returned to being laid-back," Stacey recalls, and both felt freer to enjoy sex more. She began initiating it in new ways.

"One night I came home from work, and Stacey called me into the bedroom," Randolph says. "She was wearing lingerie, and the room had low light and candles. She had never done that before, and I definitely wasn't expecting it. It blew my mind!"